1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus to be used for electrophotographic image formation such as a copier, electrostatic printing, a facsimile, a printer, and electrostatic recording.
2. Description of the Related Art
In image formation by electrophotography, an electrostatic charge image (latent image) is formed on an electrostatic latent image bearing member, the latent image is developed by use of a charged toner to form a toner image, and then the toner image is transferred onto a recording medium such as paper, and fixed by a method such as heating to obtain an output image.
Recently, image forming apparatuses by electrophotography have come to be used also in the field of commercial printing, so-called production printing, and image forming apparatuses that are higher in speed and capable of forming high-quality full-color images have been demanded.
One of the important challenges in obtaining a high-quality full-color image is to continuously supply a toner amount according to a desired image density onto an electrostatic latent image bearing member in order to reproduce a latent image on the electrostatic latent image bearing member exactly by toner.
For example, in terms of a one-component developing system, a phenomenon (fading phenomenon) has been reported in which a band-like part with a low image density is produced when the same image patterns are continuously output. This fading phenomenon occurs mainly because a low-charged toner, due to friction with the surface of a developer bearing member (developing sleeve), slips through a concentrated magnetic field formed by magnets in the developing sleeve and a blade (doctor blade) for regulating the toner layer thickness to be conveyed to a developing region as a part of a toner layer, and does not move onto an electrostatic latent image bearing member even when having received a developing electric field. Therefore, in Japanese Patent No. 3005081 and Japanese Patent No. 3126433, by using an image forming apparatus composed of a developing sleeve having a surface layer with an inclination γ of 10 or more in a work function measurement, formed of a resin layer containing conductive fine particles or a solid lubricant and toner whose weight-average particle diameter, fine powder content by percentage, coarse powder content by percentage, and MI (melt index) value have been controlled to specific ranges or an image forming apparatus composed of the developing sleeve and toner having a silicone oil- or silicone varnish-treated external additive, the toner can be stably charged to a desired value even in a high-temperature and high-humidity environment, and conveying to a developing region only a toner with an appropriate charge amount not by friction between the toner and developing sleeve surface but by an image force acting between the toner and developing sleeve prevents a fading phenomenon.
Also in terms of a two-component developing system more suitable for a higher-speed image forming apparatus, it has been reported that, when a developer is used for a long period of time, a hysteresis occurs in which the developing performance declines to reduce image density (Japanese Patent Application Laid-Open (JP-A) No. 11-065247). The hysteresis in the two-component developing system disclosed therein is caused by the fact that releasing of a two-component developer is not normally performed. Releasing of the developer is performed by providing magnets in odd numbers in a developing sleeve and providing a magnet pair of the same polarity at a position lower than the rotating axis of the developing sleeve to form a releasing region that has nearly zero magnetic force, and causing the developer after development to naturally fall using gravity in the region. However, as a result of a counter charge being generated in a carrier during toner consumption for a preceding image, an image force is generated between the carrier and developer bearing member, and the developer is not released normally. Therefore, the developer with a toner concentration lowered due to toner consumption is again conveyed to the developing region, and the developing performance declines. That is, there is a problem that the image density is normal for one round of the sleeve, whereas the second round onward results in a low image density. To cope therewith, JP-A No. 11-065247 mentioned above has proposed a method in which a draw-up roll having magnets inside is disposed near a releasing region on the developing sleeve, and releasing of the developer after development is performed by means of a magnetic force thereof. The released developer is drawn up by another draw-up roll and then conveyed to a developer stirring chamber having screws, and a re-adjustment of the toner concentration and toner charging are therein performed. However, in the above-described proposal, there has been a problem that an initial hysteresis can indeed be eliminated, but in the case of continuous use over time, a sufficient effect cannot be exerted, and a hysteresis occurs.
The present inventors have discovered, in the course of studying an image forming apparatus that is high speed and capable of forming high-quality full-color images, there is a problem, as another hysteresis in the two-component developing system, that a toner remaining without being developed in the developing region is not collected together with the carrier, and remains adhered on the developer bearing member, and when the remaining toner is again conveyed as it is to the developing region together with a newly drawn-up developer at the next development, an image density difference occurs depending on whether there is remaining toner on the developer bearing member. This hysteresis is considered to occur when the adhesion force between the toner and developer bearing member has become greater than the adhesion force between the toner and carrier. This tendency becomes prominent when a developer is continuously stirred over time under conditions where the toner consumption is small, and becomes more prominent in the case of, particularly, a high-speed machine. Moreover, in response to the recent demand for energy saving, low-temperature fixing of toner has been promoted, and as one of the means therefor, there have been made many proposals to add into toner a crystalline resin (particularly, a crystalline polyester resin) that indicates a sharp melt property to the temperature, but this hysteresis tends to be more prominent in an image forming apparatus using such a low-temperature fixing toner.